In the mammal, the anterior pituitary gland is located at the base of the brain, but is separate from it. A special set of closed circulation blood vessels connect the anterior pituitary to the brain at the region of the hypothalamus. It is the activity of the hypothalamus which largely regulates the production of luteinizing hormone, (LH), and follicle stimulating hormone, (FSH), by the anterior pituitary.
Within the hypothalamus, neurosecretory cells manufacture and release gonadotropic releasing hormones such as luteinizing hormone releasing hormone, (LHRH), also know as gonadotropic releasing hormone, (GnRH). LHRH enters a closed system of blood vessels directly connecting the hypothalamus with the anterior pituitary. As LHRH contacts neurosecretory cells located within the anterior pituitary these cells are stimulated to release luteinizing hormone into the systemic blood stream. In a similar manner, the hypothalamus causes the anterior pituitary to release FSH.
A developing mammalian egg, an oocyte grows to maturation within an ovarian follicle. Cyclically, the hypothalamus secretes follicle stimulating hormone releasing factor into the closed capillary system attaching the hypothalamus to the anterior pituitary. Once FSHRH contacts the anterior pituitary, it stimulates neurosecretory cells to produce FSH. FSH causes the mammalian follicle to grow both in size and number of cells. The follicle cells in turn secrete estrogen which stimulates the growth of the uterine wall in preparation for implantation of an embryo should fertilization occur. A feedback phenomenon occurs as estrogen level production stimulated by FSH rise causing both a direct reduction in the output of FSH by the anterior pituitary as well as an indirect effect by means of reducing hypothalamic stimulus of the anterior pituitary.
As the follicle reaches full maturity, the hypothalamus responds to the rising estrogen levels by secreting LHRH or luteinizing hormone releasing hormone into the closed capillary system connecting the hypothalamus with the anterior pituitary. As LHRH reaches the anterior pituitary, it stimulates release of luteinizing hormone. Luteinizing hormone stimulates the completion of maturation of the follicle and ovum. LH is also known as interstitial cell-stimulating hormone since it acts upon the interstitial cells of the testes in stimulating production of testosterone. After the mature follicle has released an ovum into the oviduct, the corpus luteum, which is derived from the remnant granulosa and theca cell of the ruptured follicle cells, becomes the equivalent of an endocrine gland secreting progesterone under the influence of LH.
If a fertilized egg is implanted, chorionic gonadotropin or CG is secreted by the placental tissues. CG prevents the corpus luteum from degenerating and allows it to continue its production of progesterone. Progesterone maintains the growth of cells of the endometrium as well as maintaining an adequate blood supply to nourish an implanted embryo.
Normally, as outlined above, the function of LH and FSH are biologically positive. FSH stimulates the mammalian follicle to produce estrogens while LH stimulates the corpus luteum to produce progesterone and the interstitial cells of the testes and ovaries to produce testosterone and estrogen respectively. FSH and LH have a synergism. That is to say, LH, when administered by itself has little or no effect, but combined with a small dose of FSH induces follicular maturation. Likewise, a small amount of LH greatly augments the response of the response of tissue to a small amount of FSH. For this reason, LHRH antagonists also affect the activity of FSH.
The above discussed hormones may be classified as gonadotropic as they stimulate growth and function of reproductive tissue. However, there are certain situations in which the gonadotropic effects of these hormones may deleteriously affect the health of an individual. Certain tumors derived of hormone dependent tissue are stimulated by the same gonadotropic hormones that stimulate healthy tissue. If such tumors are exposed to the normal anabolic effect of such hormones, rapid growth, and in the case of malignant tumors, metastasis is encouraged.
Various treatment modalities have been available for treating disease of hormone responsive tissue. Basically, these treatment modalities may be classified as those involving estrogen, androgen, and progestin additive therapy, or ablative procedures involving orchidectomy and removal of the ovaries.
Treatment of hormone dependent pathology such as uterine fibroids, breast, prostatic and testicular interstitial cancer, endometriosis and certain human papillomavirus associated tumors may be accomplished through altering the amount of circulating estrogen, progesterone, or testosterone. Precocious puberty may be treated by reducing the levels of circulating gonadotropic hormones.
Ablation, or castration therapy has been utilized in treating tumors derived from organs which are normally responsive, or dependent upon hormones. Ablative surgery has been used extensively in women with breast carcinoma. Removal of the ovaries is most beneficial to premenopausal women in whom there has been a long interval between mastectomy and recurrence or who have mainly osseous and soft-tissue metastases.
Orchidectomy has been utilized to treat carcinoma of the prostate. As with other ablative treatments, hormone therapy is sometimes used in place of excision. In prostate carcinoma, estrogen therapy has been utilized with some measure of success. Castration is especially effective in men with breast cancer and results in a response rate of nearly 70 percent. Thorn et al., Harrison's Principles Of Internal Medicine, eighth ed. pp. 1753, (1977).
Filicori et al., GnRH Agonists and Antagonists Current Clinical Status, Drugs 35:63-82 (1988) discloses the clinical application of GnRH analogues. The article discloses the use of these agonist drugs in successfully treating precocious puberty, prostatic cancer, breast cancer, female contraception, male contraception, endometriosis, uterine leiomyoma, and polycystic ovarian disease. However, although promising results in treating these various pathologies are disclosed, the need for effective LHRH antagonist is strongly emphasized.
LHRH antagonist drugs of the past have required extensive modifications in the native LHRH to obtain a potent antagonistic effect as compared to the relatively minor changes required for formulating superactive GnRH agonists. It has been believed that the number and the type of amino acid substitutions and the resulting conformation of the antagonist that affects LHRH receptor binding.
There is currently much research directed to the use of gonadotropic antagonists such as LHRH antagonists in treating pathological conditions which are normally responsive to a reduction in plasma levels of gonadotropic hormones such as uterine fibroids, precocious puberty, endometriosis and hormone dependent carcinomas. These antagonist peptides strongly inhibit LH secretion and have some effect, as explained above in diminishing FSH activity.
Well known examples of LHRH antagonists are those described by A. V. Schally and others in Proc. Natl. Acad. Sci. (USA), Vol 85, pp. 1637-1641 (1988). K. Folkers and others in Tetrahedron, Vol 46, pp. 33297-3304 (1990), also in Proc. Natl. Acad. Sci. (USA) Vol 85, pp. 8236-8240 (1988).
Although LHRH antagonists offer a promising alternative in treating hormone responsive disease, peptide antagonists synthesized so far have demonstrated serious deficiencies and side effects. Presently, sufficient potency has not been demonstrated in vivo, for the antagonistic peptides. Furthermore, serious side effects such as histamine release, anaphylactoid reactions, and hypotension occur. LHRH antagonists have also caused local vascular permeability changes and associated edematogenic effects, poor water solubility and inadequate duration of action.
What is needed is a water soluble LHRH antagonist peptide which exhibits sufficient potency so as to achieve an effective therapeutic effect, while minimizing anaphylactoid reactions of past antagonists described above.